1. Field of the Invention
The present invention relates to a method for the treatment of aqueous wastes, and particularly to a method for removing phenol from waste water.
2. Background of the Art
Various chemical processes produce waste water streams which contain phenol. For example, in a commonly employed process phenol is produced by the peroxidation of cumene to cumene hydroperoxide, which is then cleaved to phenol and acetone. Alpha-methyl styrene (AMS) is also produced and can be recovered as a by-product, or hydrogenated to cumene and then recycled to the phenol process. Waste water streams from such a process contain dissolved phenol which must be removed before the waste water can be discharged into the environment.
Various methods have been used in the past to remove phenolics (phenol and cresol) from waste water streams, including chemical reaction, adsorption with resins, and liquid-liquid extraction.
In extraction processes the phenol-containing water is treated with an extractant. Typically, the extractant is a solvent which is immiscible in water, but in which the phenol preferably dissolves. The phenol is thereafter recovered from the extractant stream. Various extractants have been employed, including benzene, toluene, xylene, cumene, and other aromatic or aliphatic liquids.
For example, U.S. Pat. No. 3,963,610 to Hauschulz et al. discloses a method of removing phenol from wastewater using cumene as an extractant. The cumene is washed with aqueous sodium carbonate and then sodium hydroxide to recover the phenol as a phenate.
U.S. Pat. No. 6,071,409 to Bondy et al. discloses phenolic wastewater treatment with ethers for removal and recovery of phenolics.
In a conventional method, aqueous effluent containing phenol is treated with solvent extraction to remove phenol in a single-stage extraction method which includes recycling of the solvent. The fresh solvent is supplied from an overhead of an AMS topping column and includes cumene and other hydrocarbons. The phenol is recovered from the solvent by caustic washing and is then recycled back to the process.
More particularly, referring now to FIG. 1, a prior art system 10 for the recovery of phenol and acetone from the product of the oxidation and cleavage of cumene is shown. Methods and apparatus for oxidizing cumene to cumene hydroperoxide and cleaving the cumene hydroperoxide to acetone and phenol are known. Effluent 11 from the cleavage of cumene hydroperoxide contains acetone, phenol, and by-products such as cumene, AMS, and other components (e.g., benzene, toluene, ethylbenzene, butyl benzene). The effluent 11 is sent to a splitter 12 which separates by fractionation an overhead stream 13 containing acetone, cumene and AMS, and a bottoms stream 14 containing phenol and some AMS. The bottoms 14 is sent to distillation column 21 wherein AMS is separated out as an overhead stream 22 and phenol and other heavier components are separated out as a bottoms stream 23. The bottoms stream 23 is then sent on to further purification in distillation column 24 wherein purified phenol is separated as an overhead stream 25 and heavier components are separated out as a bottoms stream 26.
The overhead stream 13 from splitter 12 is sent to distillation system 15 wherein acetone is separated out as an overhead stream 16, and cumene and AMS are separated out as a bottoms stream 17. The bottoms stream 17 is sent to the distillation system 18, which separates the cumene-AMS mixture by fractionation into an overhead stream 19 containing cumene and other components, and a bottoms stream 20 containing AMS.
The overhead from the distillation system 18 is then sent to prior known system 50 for recovery of phenol from waste water. Referring now to FIG. 2, waste water stream 52 is introduced into a primary dephenolation drum 51. The waste water stream may optionally be pre-treated with acid (e.g., H2SO4) to convert any phenates in the water to phenol, which can be removed by the hydrocarbon extractant. Stream 19 containing cumene and other hydrocarbons is introduced into drum 51 and the dephenolated water is discharged via stream 53. The hydrocarbon solvent stream, which contains a major portion of the phenol, is sent via stream 55 to caustic wash drum 56. Caustic solution (e.g., 20% aqueous NaOH) is added to the drum via stream 57 to convert phenol back into a phenate (sodium phenate) which is preferentially soluble in water and thereafter removed in water stream 58. A portion of stream 58 may optionally be recycled to stream 57. The hydrocarbon stream, is removed via stream 59. A portion of the hydrocarbon solvent is recycled via stream 54 back to stream 19. Another portion 60 is sent on to further processing.
What is needed is an improved method for extracting phenol from a wastewater stream which can be used in conjunction with a process for producing phenol and acetone from cumene.
A method is provided herein for the extraction of phenol from a waste water stream. The method is used in conjunction with a process for producing acetone and phenol from cumene which includes an acetone finishing column and an alpha-methylstyrene recovery system downstream of the acetone finishing column. The method for extracting phenol from waste water comprises contacting a phenol-containing waste water stream with a fresh hydrocarbon solvent, the hydrocarbon solvent being a hydrocarbon portion of a bottoms stream from the acetone finishing column, to produce a dephenolated waste water stream and a spent hydrocarbon solvent; and, conveying at least some of the spent hydrocarbon solvent to the alpha-methylstyrene recovery system without recycling any portion of the spent hydrocarbon solvent to the step (a) of contacting the phenol-containing waste water. Preferably, a major portion of phenol in the spent hydrocarbon is removed, for example by caustic washing, prior to step of conveying the spent hydrocarbon solvent to the alpha-methylstyrene recovery system.
Preferably the fresh hydrocarbon stream derived from the bottoms stream of the acetone finishing column is water washed prior to contacting the phenol-containing waste water stream in order to remove any entrained phenate.
The method and system of the present invention advantageously provides significantly better phenol recovery from the waste water than the prior known method.